One method for efficiently using the capacity of a storage system is a hierarchical storage management (HSM) method. Management targets of the HSM are generally two or more storage apparatuses including a high-speed, small-capacity storage apparatus and a low-speed, large-capacity storage apparatus and there is a mutual hierarchical relationship between the storage apparatuses which are management targets.
If a hierarchical layer(s) is referred to as a tier(s), the high-speed, small-capacity storage tier may be referred to as the high-order tier (Tier 1) and the low-speed, large-capacity storage tier may be referred to as the low-order tier (Tier 2). When managing the hierarchical file storage apparatuses by the HSM method, sufficient utilization of the storage capacity can be ensured by firstly storing data in the high-order tier and then migrating data whose use frequency is low, from among the data stored in the high-order tier, to the low-order tier.
With conventional hierarchical file storage apparatuses, a file storage apparatus belonging to the high-order tier and a file storage apparatus belonging to the low-order tier, from among a plurality of file storage apparatuses, are connected to each other via a WAN (Wide Area Network) or a LAN (Local Area Network) and migration target files designated according to, for example, policies are migrated from the high-order tier to the low-order tier.
If a plurality of file systems exist in the file storage apparatus in each tier under the circumstance described above, file migrations are performed from a plurality of file systems belonging to the high-order tier to a plurality of file systems belonging to the low-order tier during a time period designated by the user (generally a time period such as late at night when the migrations will have little effect on the performance of services provided to the user).
The user can access files through a file sharing service that operates on the file storage apparatus in the high-order tier. In this case, a technique for accessing files existing in the low-order tier via a stub located in the high-order tier is disclosed in Patent Literature 1.
Furthermore, regarding a method for predicting the execution of migrations, a method for predicting the execution of migrations based on load on the high-order tier is suggested as disclosed in Patent Literature 2.
Citation List
Patent Literature
    PTL 1: U.S. Pat. No. 7,225,211    PTL 2: U.S. Pat. No. 7,594,076